As a result of military testing programs involving the detonation of nuclear devices, both in the United States and abroad, the environment, and particularly vast areas of soil in testing zones have become contaminated with nuclear waste materials. In some instances, for example, detonation of a nuclear device failed to achieve the needed critical mass of the radioactive components, resulting in substantial quantities of enriched uranium and plutonium being scattered over wide areas of desert testing grounds. In addition to nuclear testing programs, contamination of soil with radioactive materials has occurred at nuclear weapon manufacturing sites, such as at Hanford, Wash.; Rocky Flats, Colo.; Savannah River, Ga.; Oak Ridge, Tenn., and elsewhere through spills or releases into the environment.
Efforts to successfully decontaminate these sites have proven difficult and extremely costly due to massive amounts of soil requiring treatment and/or storage. Cleanup has usually meant a slow and costly process where the contaminated soil is excavated and transferred to a different location for storage. Abandoned salt mines and mountain repositories have been proposed as storage facilities for nuclear wastes, but too often rejected later on for technical and/or political reasons. Because of a finite amount of space available for storage of nuclear waste materials progress in the reclamation of contaminated sites has been slow.
In an effort to mitigate the nuclear waste storage crisis systems for reducing bulk quantities of contaminated soil requiring storage have been proposed wherein the radioactive components are concentrated in a soil fraction. One system, for example, employs an aqueous washing process requiring the use of soil scrubbing chemicals, multiple separation steps, water treatment, and so on. Although quite effective in concentrating radioactive components in silt and clay fractions of soil, capital and operating costs per ton of soil treated are viewed as economically unattractive. Consequently, most methods proposed for concentrating nuclear waste have not received wide acceptance.
Like nuclear wastes, the contamination of soil with elemental mercury also poses serious health risks to humans, threatens serious harm to wild life and presents a long term threat to the environment. Instances of mercury contamination of the environment are well documented. One representative source of mercury contamination has been through the use of mercury electrolysis cells in the synthesis of chlorine and caustic soda. The mercury cell employs a mercury cathode so that the sodium metal produced at the cathode reacts promptly with the mercury to form an amalgam, NaHg, thus being separated from other products. Posterior treatment with water converts the NaHg amalgam into caustic soda, hydrogen and mercury metal, the latter of which is recycled for further use. While this type of electrolysis cell has been gradually withdrawn as an industrial process because of pollution of the environment with mercury, other applications of mercury besides electrodes have included photography, electric switches, control apparatuses, catalysts, and so on. They have contributed to contamination of lakes, oceans and soil. Decontamination of the environment of metallic mercury and its recovery has been a major problem.
Other related environmental problems have been the treatment of soil contaminated with "mixed wastes." The expression "mixed wastes" denotes wastes containing two or more separate classes of contaminates requiring destruction or removal from the environment. One representative example of soil contaminated with mixed wastes has been PCB-contaminated materials, such as dielectric fluids discharged onto soils also contaminated with elemental mercury. Together, they pose an especially difficult problem for treatment using conventional technologies. While mercury bearing soils might be land filled, the presence of PCBs eliminates land filling in anything other then a PCB-approved landfill, which is not permitted for other wastes. Therefore, land filling is normally not permitted. Conversely, PCB-contaminated soils can be incinerated, but the presence of mercury precludes this option because oxides of mercury formed during incineration are hazardous when released to the atmosphere. This dilemma has resulted in mixed wastes historically commanding a substantial premium for their treatment.
Accordingly, there is need for an innovative, cost-effective process for decontaminating soils containing nuclear waste materials, such as those generated at sites of nuclear weapon plants, nuclear testing sites, and wherever treatment calls for managing substantial volumes of soil contaminated with radioactive materials. The process should enable reduction of the space otherwise required for storage of untreated soils by concentrating in a small fraction of the soil while also permitting reclamation of these sites. Likewise, a mechanism is needed whereby elemental mercury and mixed wastes-containing mercury and other contaminants, such as organics like pesticides, dioxins, PCBs and/or nuclear wastes like radionuclides can be readily separated from soil for subsequent recovery and proper disposal.